Bump to 8.4.0-SNAPSHOT for test

Author: ezimuel

.ci/test-matrix.yml

@@ -1,6 +1,6 @@
 ---
 STACK_VERSION:
-  - 8.3.0-SNAPSHOT
+  - 8.4.0-SNAPSHOT
 
 PHP_VERSION:
   - 8.1-cli

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
       matrix:
         php-version: [7.4, 8.0, 8.1]
         os: [ubuntu-latest]
-        es-version: [8.3.0-SNAPSHOT]
+        es-version: [8.4.0-SNAPSHOT]
 
     steps:
     - name: Checkout